Electrophoretic deposition of organic resins is a known technique for coating conductive substrates. It is a common technique for applying protective coatings to prevent rust and corrosion of metal.
In the electrocoating technique, solubilized resins having a positive or negative electrical charge are placed in an appropriate container usually in the form of an aqueous solution, the resulting mixture is referred to in the art as an "e-coat" bath. An opposite charge to that of the resin is applied to a conductive substrate which is placed in the bath contacting the resin solution. At the point of contact, the charge on the resin is neutralized and the resin precipitates out of solution thereby coating the conductive substrate. The substrate is then removed from the bath, rinsed and subjected to a curing procedure.
Although electrocoating is an effective method to coat conductive materials, there has consistently been a need to increase the thickness of coatings applied by the method. The thickness is limited because the conductive substrate becomes progressively insulated by the film being deposited onto it, thus reducing the attractive forces between charged resin molecules and charged substrate, until no further deposition takes place. In U.S. Pat. No. 4,487,674 assigned to the same assignee now issued as U.S. Pat. No. 4,487,674, hereby incorporated by reference it was disclosed that thicker coatings could be achieved by adding suitable surfactant to the e-coat system in concentration ranges greater than those employed in standard industry practice. While the use of increased surfactant allows increased film thickness, it was found that increased surfactant concentration was not a total solution for achieving thicker film because the resulting films had certain deficiencies such as softness, pinholes, etc.
Coating quality is as important as thickness because both factors affect the degree of protection afforded to the substrate by the coating. Thus, it is desirable that increased thickness be achieved together with good film quality. Characteristics considered in evaluating film quality include appearance, adhesion, hardness and solvent resistance. It has now been found that film coatings of increased thickness that retain high quality can be achieved with addition of a new additive mixture to the e-coat system. The new additive includes a surfactant in higher concentration than employed in standard industry practice, a reactive viscosity modifier, and a solvent. The new additive mixture has been shown to enable e-coat procedures to deposit up to double the thickness previously achievable, while still allowing deposited coating to possess the desired characteristics of good appearance, adhesion, hardness, and solvent resistance.